This invention relates to blends of linear low density polyethylene (LLDPE) and polypropylene (PP) and to a method for improving the processability of linear low density polyethylene (LLDPE) in extruding films by blending a minor amount of polypropylene (PP) having a moderate amount of atacticity with the LLDPE and extruding the blend to obtain a blown film.
Linear low and medium density polyethylene polymers are widely used commercially in films which are conventionally prepared by blown film extrusion. Because of the structure and rheology of LLDPE, it is more viscous at extrusion shear rates and requires more power to extrude than low density polyethylene (LDPE). A wide die gap has also generally been used in extrusion to avoid melt fracture. It has also been known that polypropylene can improve the stiffness of LLDPE. Adding polypropylene, however, has been known to adversely affect impact properties and tear strength of LLDPE films.
U.S. Pat. No. 4,871,813 describes blown films with good hot tack strength during heat sealing, made with a composition of 99 to 75 weight percent LLDPE and 1 to 25 weight percent of polypropylene copolymers having a fusion enthalpy equal to or lower than 75 J/g, that is, a degree of crystallinity or isotacticity lower than 35%, and a degree of atacticity higher than 65%. The required polypropylene copolymers are highly specific and the use of polypropylene homopolymer to improve processability of LLDPE is not suggested. The fusion enthalpy of the copolymer is described as a characteristic connected with the molecular structure of the copolymer, higher in proportion as the structure is more ordered so that the fusion enthalpy of a purely isotactic polypropylene would be 210 J/g. In contrast, in the present invention, the PP has a much higher isotacticity of about 70-90%.
U.S. Pat. No. 4,565,847 describes a blend of LLDPE, PP and ethylene-propylene diene monomer copolymer (EPDM) or ethylene-propylene rubber (EPR). When LLDPE/PP alone was used to prepare films, the films exhibited a catastrophic decrease in impact and tear strength, particularly in the machine direction (MD).
U.S. Pat. No. 3,254,139 describes wire coating blends of linear polypropylene and linear ethylene polymers. The polyethylenes are low density branched polyethylenes or high density unbranched polyethylenes. There is no suggestion of LLDPE or that isotacticity of polypropylene is a factor which requires careful selection. In addition, in wire coatings, properties such as impact and stiffness are not important. In contrast, it has now been found that to attain a desired balance of extrudability and property enhancement, the correct isotacticity index of polypropylene is an important factor particularly in blown films where extrudability, impact and stiffness are of great significance.
U.S. Pat. No. 3,426,105 describes a blow molding composition of a high density polyethylene (HDPE) copolymer of ethylene and 1-butene having a density of 0.945 to 0.955 and a polypropylene having a density of about 0.903 to 0.915. The described composition is useful for molded articles and is unsuitable for blown films.
Japan Patent No. 49041342-A describes a blow molding composition containing LLDPE and polypropylene having a ratio of the melt flow index of polypropylene to the melt flow index of polyethylene between 1.0 and 3.0. Because of the disclosed melt indices, the blow molding composition described in the Japan patent would have a viscosity which would make it unsuitable for blown films. It is known that the melt index indicates the viscosity of a thermoplastic polymer at a specified temperature and pressure. Specifically, the melt index may be generally defined as the number of grams of the polymer which can be forced through a 0.0825 inch orifice in 10 minutes at 190.degree. C. by a pressure of 2160 g. Blow molding as in the Japan patent is generally used to form rigid hollow objects such as bottles. Rigid articles require higher viscosity materials.
The prior art discussed above has suggested various kinds of blends of polyethylene and propylene but all of the described blends are unsuitable for improving the processability of blown films. In addition, various methods have been suggested for improving the processability and properties of blown films using ternary blends in which other components have been necessary in addition to LLDPE and PP. When attempts were made to use LLDPE and PP alone for improving the properties and processability of blown film blends, the results described in the prior art were not advantageous.
According, a method for improving the processability of blown films without necessity of forming ternary blends would be economically advantageous.
It is an object of the invention to improve the processability of LLDPE blown films by a judicious selection of the properties of the polypropylene and LLDPE components of a film blend.
It is a specific object of the invention to provide a blend which can be extruded into blown films with a minimum of pressure while yielding films with good properties.